The present invention relates to a wave arrival direction estimating method and an antenna apparatus having a wave arrival direction estimating function, to be applied to a radar, a mobile communication base station, and the like.
This application is based on Japanese Patent Application No. 10-372740, filed Dec. 28, 1998, the entire content of which is incorporated herein by reference.
A technique of estimating a wave arrival direction has been used for various kinds of controls, such as, for example, a tracing of a flying object or a detecting of its arrival direction by radar, a control of directing a beam pattern to a specific mobile station or, on the contrary, not directing a beam pattern to a specific mobile station, from an antenna at a base station of a mobile radio communication system, and a detection of an arrival direction of an interference wave and forming a null of a beam pattern to this direction, and the like.
Among such techniques for estimating a wave arrival direction, there has been known MUSIC (Multiple Signal Classification) method, particularly as a high-resolution estimating technique capable of simultaneously estimating arrival directions of a plurality of waves of the same frequency. Details of this MUSIC are described in R. O. Schmit, xe2x80x9cMultiple Emitter Location and Signal Parameter Estimationxe2x80x9d, IEEE Trans. Antennas and Propagation, vol. AP-34, no. 3, pp. 276-280, March, 1986 (Document 1).
The MUSIC is an algorithm for estimating wave arrival directions by the following processes. At first, high-frequency signals output from antenna elements constituting an array antenna are input to receivers. Then, a covariance matrix of reception signals from the receiver is obtained, and an eigen value and an eigen vector of this matrix are calculated. By comparing sizes of the eigen value and noise power, the eigen vector is divided into a signal subspace and a noise subspace. Wave arrival directions are estimated by utilizing the orthogonality of the signal subspace and the noise subspace. This method is characterized in that the method is generally called a high-resolution method as the wave arrival direction is estimated by utilizing the null of the beam pattern, and that it is possible to simultaneously estimate directions of arrival waves by the number of xe2x80x9cthe number of antenna elements minus onexe2x80x9d.
However, according to the MUSIC, in order to simultaneously estimate the directions of a plurality of arrival waves, it is necessary to prepare receivers for receiving and demodulating high-frequency signals output from the antenna elements to obtain reception signals by the number of antenna elements. For example, in the case of applying the MUSIC under a propagation environment where the number of arrival waves is unknown, it is necessary to prepare a sufficient number of antenna elements. Accordingly, it is also necessary to provide receivers by the same number as that of the antenna elements. As a result, the size of a wave arrival direction estimating apparatus becomes large, leading to a higher cost of the apparatus.
When the number of the antenna elements increases, the order of the covariance matrix to be used for estimating the wave arrival directions becomes larger. As the covariance matrix is obtained from an average of a large number of reception signals, the volume of data becomes larger and it also takes a long processing time. Further, when the order of the matrix becomes larger at the time of calculating the eigen value of this covariance matrix, the time required for the calculation increases in proportion to the cube of the order. Taking a long time for the calculation becomes a serious problem particularly in the case of estimating the wave arrival direction in on-line processing.
As means for solving the above-described problems of the MUSIC, there have already been proposed the following two techniques.
A first method provides the following processing. As shown in FIG. 1, a high-frequency switch 202 sequentially selects high-frequency signals output from antenna elements 201. Only one high-frequency signal is input to one receiver 203 at one time. A reception signal from the receiver 203 is guided to an arrival direction estimation circuit 204, and a calculation similar to that of the MUSIC is carried out. Details of this method are described in SEKIZAWA, xe2x80x9cDevelopment of Linear Array Antenna for Estimating Arrival Directions of Multiple-wavesxe2x80x9d, Singaku-Giho, RCS96-128, pp. 7-14, January, 1997 (Document 2).
According to this method, it is possible to provide a small apparatus at low cost, as only one receiver 203 is required. However, as the high-frequency signals output from the antenna elements 201 are sequentially demodulated in time series by the receiver 203, it is not possible to carry out a precise estimation of the arrival directions when the propagation environment has changed during this processing. Particularly, this problem becomes serious when the number of the antenna elements 201 increases. Further, according to this method, the calculation itself is basically similar to that of the MUSIC, and it is not possible to improve the processing time.
A second method is to use a technique called beam space MUSIC (hereinafter to be referred to as BS-MUSIC). Details of this BS-MUSIC are described in HARRYB. LEE and MICHAELS. WENGROVITZ, xe2x80x9cResolution Threshold of Beamspace MUSIC For Two Closely Spaced Emittersxe2x80x9d, IEE Trans. Acoust. Speech and Signal Processing, vol. ASSP-38, no. 9, pp. 1545-1559, September, 1990 (Document 3).
The BS-MUSIC will be explained briefly with reference to FIG. 2. High-frequency signals output from antenna elements 211 are simultaneously input to a beam synthesis circuit 212, to form beams of a plurality of antennas. The high-frequency signals for the beams are sequentially received by a receiver 213, and a wave arrival direction is estimated by an arrival direction estimation circuit 214 based on the reception signals. The difference of algorithms between the MUSIC and the BS-MUSIC is that, according to the BS-MUSIC, a calculation expression includes a weight vector multiplied to a high-frequency signal from each antenna element 211 when an antenna beam has been formed. Basically, the calculation procedure is similar to that of the MUSIC. According to the BS-MUSIC, as shown in FIG. 2, only one receiver 213 is necessary in a similar manner to that of the method of the Document 2, and it is possible to provide a small apparatus at low cost.
However, according to the BS-MUSIC, the arrival waves from the directions to which the antenna beams are directed are received by main lobes of the beams, but the arrival waves from the directions to which the antenna beams are not directed are received by side lobes of the beams. This means that the arrival waves received by the side lobes are not substantially received. In other words, while it is possible to estimate the wave arrival directions for the waves to which the beams are directed by the beam synthesis circuit 212, it is not possible to estimate the wave arrival direction for the waves to which the beams are not directed. However, in the case of applying the BS-MUSIC under the unknown propagation environment, it is necessary to direct antenna beams to all directions in order to estimate the an arrival direction of the wave from all directions. Further, the number of arrival waves that can be separated in the BS-MUSIC is xe2x80x9cthe number of beams formed by the beam synthesis circuit minus onexe2x80x9d, and this number also has a limitation like that of the MUSIC.
Accordingly, in order to carry out a precise estimation of wave arrival direction in the BS-MUSIC under the environment having a potential of existence of a large number of arrival waves, it is necessary to form a large number of antenna beams by using a large number of antenna elements assuming a large number of arrivals of waves. Thus, it is not possible to solve the second problem that it takes a long processing time.
As described above, according to the wave arrival direction estimating technique using the prior-art MUSIC method, a large number of receivers are necessary, which has a problem of an increase in the size of the arrival direction estimating apparatus, leading to an increase in the cost of the apparatus. Further, when a large number of antenna elements are used in order to precisely estimate wave arrival direction under the propagation environment where the number of arrival waves is unknown, it takes a long time for the calculation, making it possible to carry out an on-line processing.
Further, according to the BS-MUSIC technique requiring only one receiver through the improvement of the MUSIC technique, as disclosed in FIG. 1 and FIG. 2, it is also necessary to use a large number of antenna elements for precisely estimating wave arrival direction under the environment with a potential of existence of a large number of arrival waves. Thus, it is not possible to solve the problem that it takes a long processing time.
The present invention has been made to cope with the above-described situation, and it is an object of the invention to provide a wave arrival direction estimating method capable of providing a compact and low-cost wave direction estimating apparatus with a small number of receivers and capable of estimating arrival direction in a small volume of calculation even under the unknown environment of waves.
It is another object of the present invention to provide an antenna apparatus having a wave arrival direction estimating function according to the above estimating method.
In order to achieve the above objects, in a first aspect of the present invention, there is provided a wave arrival direction estimating method for receiving an arrival wave with a plurality of antenna elements arrayed in a predetermined manner, inputting high-frequency signals obtained by this reception into one receiver, and estimating an arrival direction of the wave from reception signals generated, wherein the arrival direction estimating method includes a first arrival direction estimation wherein a first wide antenna beam is formed and radiated by a plurality of times in plural directions, and a second arrival direction estimation wherein a second antenna beam with a smaller width than the first antenna beam is formed and radiated by a plurality of times in directions of a result of the first arrival direction estimation.
In other words, in the first arrival direction estimation, a rough estimation is carried out by using the wide antenna beams radiated in plural directions. In the second arrival direction estimation, an antenna beam with a smaller beam width is formed by plural times and radiated in the directions of a result of estimation by the first arrival direction estimation, thereby to carry out a precise estimation.
According to the wave arrival direction estimating method of the above first aspect of the invention, it is possible to estimate an arrival direction in a relatively simple hardware configuration using one receiver and in a small volume of calculation even under the unknown environment of waves. For example, in the BS-MUSIC, it is possible to estimate an arrival direction by using a small number of data and a small matrix.
According to a second aspect of the invention, there is provided an antenna apparatus having a wave arrival direction estimating function according to the present invention, the antenna apparatus including a plurality of antenna elements arranged in a predetermined array which receives an arrival wave and outputting high-frequency signals, a plurality of phase shifters which individually phase-shifts high-frequency signals output from the plurality of antenna elements, a selector which selects antenna elements for receiving the arrival wave from the plurality of antenna elements, a signal synthesis circuit which synthesizes high-frequency signals phase-shifted by the phase shifters and sent from the antenna elements selected by the selector, a receiver which receives high-frequency signals output from the signal synthesis circuit and for generating reception signals, an arrival direction estimation circuit for estimating an arrival direction of the wave based on the reception signals, and a controller for controlling phase shift quantities of the phase shifters and a selection by the selector.
Further in a third aspect of the invention, there is provided another antenna apparatus having a wave arrival direction estimating function according to the present invention, the antenna apparatus being structured by using a reflection-type phased array antenna, and including a plurality of antenna elements arranged in a predetermined array, which receives an arrival wave and outputs high-frequency signals, a plurality of phase shifters for individually phase-shifting high-frequency signals output from the plurality of antenna elements, a reflector which selectively reflects some of the high-frequency signals phase-shifted by the phase shifters, thereby to re-transmit some of the signals as waves from the antenna elements, a primary emission antenna for receiving waves re-transmitted from the antenna elements and for outputting high-frequency signals, a receiver for receiving high-frequency signals output from the primary emission antenna and for generating reception signals, an arrival direction estimation circuit which estimates an arrival direction of the wave based on the reception signals, and a controller for controlling phase shift quantities of the phase shifters and a selection by the selector.
According to the antenna apparatuses of the second and third aspects of the invention, the arrival direction estimation circuit carries out a first estimation of arrival direction wherein a part of a plurality of antenna elements are selected by the selector and a predetermined number of antenna beams are sequentially formed by controlling the phase shift quantity of the phase shifters. Next, the arrival direction estimation circuit carries out a second estimation of arrival direction wherein all the plurality of antenna elements are selected by the selector and at least two antenna beams are sequentially formed by controlling the phase shift quantity of the phase shifters.
With this arrangement, it is possible to carry out a precise estimation of arrival direction in a relatively simple hardware configuration using one receiver and in a small volume of calculation even under the unknown environment of the wave.
In this case, in order to select antenna elements for receiving an arrival wave from the plurality of antenna elements, the selector consists of, for example, high-frequency switches of the same number as the antenna elements, or variable gain amplifiers of the same number as the antenna elements. The on/off of the high-frequency switches or the gain of the variable gain amplifiers is individually controlled by the controller.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.